If that were the case they'd still be flying them. While I miss the DC10's, there are many economic reasons that they are being phased out. Hell, CO, AA, UA all dumped them years ago.

Another thing about Hawaii is that a large number of passengers in both F and Y are on award tickets. I flew SFO-HNL 2 weeks ago and chatted a good bit with an f/a w/ 37 years in who only flies HNL routes. She stated that at least 1/3 of the pax on our particular flight were mileage or non-revs. I have read before that Hawaii flights for most airlines are at best break even, however service must be retained due to prestige of the carriers route network and that their customers want to fly there.

You're also assuming 100% loads. I have heard that SEA keeps the DC10 on the HNL runs due to a contract with agencies that book Hawaii vacation packages and guarantees NW a certain number of seats. These are definetly not $600 fares.